My invention comprises improvements in power steering systems of the kind shown, for example, in U.S. Pat. No. 3,877,540. Also of interest are U.S. Pat. Nos. 3,690,400; 3,692,137; 3,744,515 and 3,777,839, which illustrate the state of the art.
It is common practice in power steering gear mechanisms of the kind disclosed in this specification to use a steering shaft that is connected by means of a torsion rod to a steering pinion or steering sector gear in a vehicle steering system by means of a torsion rod. A rotary valve comprising an inner valve member and a surrounding valve sleeve is located in a fluid circuit between a fluid motor and a power steering pump for establishing a variable pressure in the fluid motor, the output or driven member of which is connected to the steering shaft of the steering system to provide a power assist to the manual steering effort of the driver of the vehicle. As torque is applied to the steering shaft, the inner valve member, which is connected to the steering shaft, is displaced angularly with respect to the valve sleeve, the latter being connected to the steering pinion or sector gear.
It is desirable in arrangements of this kind to provide a reduced steering effort when the vehicle is operated at low speeds, as is the case when parking the vehicle, and to provide an increased steering effort when the vehicle is operated under straight-ahead driving conditions at high speeds. For this purpose reaction cylinders have been provided to resist relative displacement of the inner valve member with respect to its cooperating sleeve. Such devices are shown, for example, in U.S. Pat. Nos. 3,690,400; 3,692,137; 3,744,515 and 3,777,839. These devices comprise a reaction piston acting within a reaction pressure chamber, and the pressure that is made available to the reaction chamber is dependent upon speed thus providing relatively effortless steering performance at low speeds and providing a stabilized steering performance during relatively high speed driving. U.S. Pat. No. 3,877,540 expands upon this early concept by providing a zero power assist when the valve members are in a relatively centered position, one with respect to the other, and that zero power assist condition remains until the resisting torque established by a speed sensitive pressure applied to the reaction pistons is overcome thereby allowing the steering valve to assume a relatively displaced position and to initiate a power assist operating mode.